The present invention relates to a solder ball for use in bump connection in semiconductor devices, etc., and a method for producing such a solder ball, particularly to a solder ball having high sphericity, smooth surface and high dimension accuracy and a method for producing such a solder ball.
Solder balls have conventionally been produced by an in-oil spheroidizing method. The in-oil spheroidizing method of solder balls comprises the steps of (1) introducing fine solder pieces cut at a constant interval into an oil bath having a vertical temperature distribution, (2) heating the fine solder pieces to a temperature higher than their melting point in an upper portion of the oil bath so that they are melted to become spherical by their own surface tension, (3) precipitating the molten spherical solder to a bottom portion of the oil bath kept at a temperature lower than the melting point of the solder, and (4) solidifying them and degreasing and washing them.
Recently proposed as an alternative method to the in-oil spheroidizing method is a production method called xe2x80x9cuniform droplet-spraying method.xe2x80x9d For instance, U.S. Pat. No. 5,266,098 proposes a method for producing uniform metal particles comprising (a) melting a metal in a crucible having orifices, (b) giving pressure and vibration to the resultant metal melt to cause the melt to drop through the orifices in the form of metal melt droplets, (c) applying a positive or negative electric charge to the metal melt droplets before or after passing through the orifices to suppress the coagulation of the metal melt droplets, and (d) cooling them in vacuum or in an inert gas.
With semiconductor devices having increased number of electrodes, their mounting is widely carried out by a bump connection method utilizing fine solder balls, for instance, by BGA (ball grid array). As parts such as chips, resistors, etc., have smaller sizes due to the miniaturization and higher mounting density of equipment, fine solder balls used for their mounting are required to have higher sphericity, higher dimension accuracy, narrower dimension distribution and smoother and cleaner surface. The followings are requirements for the solder balls.
(1) Sphericity
The solder balls are required to have high sphericity. In an apparatus for mounting the solder balls onto BGA packages etc., the solder balls are conveyed by rolling. If the solder balls had poor sphericity, resulting in poor rolling, then they might stop in the apparatus, resulting in poor supply of the solder balls. This necessitates frequent adjustment of the apparatus, leading to lower production efficiency. The solder balls are thus likely required to have sphericity of 0.95 or more.
(2) Dimension Accuracy, Dimension Distribution
The solder balls are required to have high dimension accuracy and narrow dimension distribution. For instance, in the case of BGA by bump connection using fine solder balls, large numbers of fine solder balls are arranged in a lattice manner on the semiconductor device. If there were large distribution in the diameters of the solder balls, there would arise a large gap between the solder balls having small diameters and lands of the substrate at the time of reflow soldering, resulting in insufficient soldering and thus poor connection. To prevent this problem, the diameters of all the solder balls should be measured by a laser measurement apparatus.
Because mechanically cut of cold-worked materials are usually used as solder materials in the in-oil spheroidizing method, it is difficult to make the size of the solder material uniform due to wear of cutting tools.
Therefore, the in-oil spheroidizing method is disadvantageous in failing to providing uniform solder balls. In the case of solder balls of 100-1,000 xcexcm in diameter, they are required to have a dimension accuracy of within xc2x110 xcexcm, and a dimension distribution within a range of xc2x15% of the average diameter.
(3) Smooth Surface
The solder balls are required to have smooth surface. For instance, in the case of BGA by bump connection using fine solder balls, a large number of solder balls are arranged at predetermined positions in the lo semiconductor devices at a time, utilizing plurality of nozzles attracting the solder balls by vacuum. In this case, solder balls having ragged surfaces are not attracted by the nozzles, resulting in the production of poor BGA packages in which all solder balls are not necessarily arranged at predetermined positions. Further to obtain good rolling characteristics, they are required to have smooth surface. In the case of the in-oil spheroidizing method, however, smooth surface cannot be maintained if the oil deteriorates, necessitating frequent change of the oil and thus poorer production efficiency.
In the case of the conventional uniform droplet-spraying method, the produced solder balls have different surface raggedness.
(4) Clean Surface
The solder balls are required to have clean surface. If impurities remain on the surfaces of the solder balls, or if carbon-concentrated layers or oxygen-concentrated layers exist on the solder ball surfaces, carbons and oxides remain on the connected surfaces as scum when melting the solder balls for connection, resulting in poor wettability of the solder and thus poorer connection reliability. In the solder ball of the present invention, the thickness of a carbon-concentrated layer is preferably 1 nm or less, and the thickness of the surface oxygen-concentrated layer is preferably 5 nm or less.
Here, the carbon-concentrated layer is a layer in which the concentration of carbon determined by an Auger electron spectroscopy is 15 atomic % or more, and the oxygen-concentrated layer is a layer in which the concentration of oxygen determined by an Auger electron spectroscopy is 15 atomic % or more.
In the oil-spheroidizing method, the resultant solder balls are contaminated with oil, necessitating a degreasing treatment. Further, alkaline degreasing using an alkaline aqueous solution and a solvent degreasing using an organic solvent such as trichloroethylene, trichloroethane, etc., have their own disadvantages. In the case of the alkaline degreasing, alkaline ion in the alkaline aqueous solution is introduced into the semiconductor materials, making it likely to cause erroneous operations of the semiconductor devices. Also the degreasing with an organic solvent is disadvantageous in that there is no organic solvent, which is safe and does not destroy the ozone layer. Trichloroethylene is not safe for human body, and trichloroethane, Flon and Halon are internationally regulated as ozone depletion materials.
Further, with respect to the characteristics of solder balls, degreasing with trichloroethane is disadvantageous in that it provides the solder balls with poor wettability. Also, if the surface of solder balls was contaminated, oxidized layers cannot be removed even with a flux. Further, adjacent balls are coagulated by the oil at the time of mounting the solder balls, failing to achieve good mounting.
Accordingly, an object of the present invention is to provide a solder ball having high sphericity, high dimension accuracy, narrow dimension distribution and smooth and clean surface, and a method for producing such a solder ball.
The solder ball of the present invention produced by a uniform droplet-spraying method has a carbon-concentrated layer of 1 nm or less in thickness and an oxygen-concentrated layer of 5 nm or less in thickness on a surface thereof. The solder ball preferably has an average diameter of 1.2 mm or less, a dispersion of a diameter distribution of 5% or less, more preferably 3% or less, and sphericity of 0.95 or more.
The solder ball of the present invention is characterized in that an area ratio of the maximum dendrite is 80% or less of a cross section including a center of the solder ball. To obtain this metal structure, the solder ball preferably contains one or more additional elements for lowering the melting point of Sn in a total amount of 0.5-60 mass %.
The solder ball preferably contains no Pb. The solder ball of the present invention is preferably made of any one of an Snxe2x80x94Ag alloy, an Snxe2x80x94Cu alloy, an Snxe2x80x94Bi alloy, an Snxe2x80x94Zn alloy, an Snxe2x80x94Agxe2x80x94Cu alloy, an Snxe2x80x94Agxe2x80x94Bi alloy, an Snxe2x80x94Znxe2x80x94Bi alloy, a Bixe2x80x94Snxe2x80x94Ag alloy and an Snxe2x80x94Agxe2x80x94Cuxe2x80x94Bi alloy.
The first solder ball of the present invention comprises Ag and/or Cu, the balance being substantially Sn. In the solder ball of this composition, the content of Ag is 0.5-8 mass %, preferably 2-6 mass %, more preferably 1-4.5 mass %. The content of Cu is 0.5-3 mass %, more preferably 1.5-2.5 mass %, more preferably 0.5-1.0 mass %.
The second solder ball of the present invention comprises a first additional element comprising Ag in an amount of 0.5-8 mass %, preferably 1-6 mass %, more preferably 1-4.5 mass %, and/or Cu in an amount of 0.1-3 mass %, 0.2-2 mass %, more preferably 0.3-1.2 mass %, and at least one second additional element selected from the group consisting of Bi, Ge, Ni, P, Mn, Au, Pd, Pt, S, In and Sb, the balance being substantially Sn.
The content of the second additional element in the second solder ball may differ depending on the melting point and hardness thereof. In the case of a high-melting-point, low-hardness solder ball, a solder ball comprising only Ag or Cu contains the second additional element in a total amount of 0.01-0.5 mass %, preferably 0.01-0.2 mass %, while a solder ball comprising both Ag and Cu contains the second additional element in a total amount of 0.006-0.5 mass %, preferably 0.006-0.1 mass %. In the case of a low-melting-point, high-hardness solder ball, the second additional element is 1-60 mass %, preferably 2-20 mass %, more preferably 3-10 mass % in total.
The method for producing a solder ball according to the present invention comprises the steps of vibrating a melt of a solder alloy in a crucible under pressure to force the melt to drop through orifices of the crucible, permitting the melt dropping through the orifices to become spherical droplets in a non-oxidizing gas atmosphere, and rapidly solidifying them.
The non-oxidizing gas atmosphere is preferably a reducing atmosphere. The reducing atmosphere is preferably an inert gas atmosphere comprising 5-10 volume % of hydrogen.